//main-slider
		var sliderButton = $('#main-slider').find('.simple');
		$('#content2').hide();
		$('#content3').hide();
		$('#content4').hide();
		
		sliderButton.click( function() {
				sliderButton.removeClass('active');
				$(this).addClass('active');
				
				var itemNumber = $(this).attr('id');
				$('#main-slider .slider-content').fadeOut(250);
				setTimeout( function() {$('#content'+itemNumber[4]).fadeIn(250)}, 250);
		});

<div id="main-slider">
        	<div class="slider-content-wrap box round">
                <div class="slider-content" id="content1">
                    бла бла бла
                </div>
                <div class="slider-content" id="content2">
                    бла бла бла
                </div>
                <div class="slider-content" id="content3">
                    бла бла бла
                </div>
                <div class="slider-content" id="content4">
                    бла бла бла
                </div>
            </div>
            <div class="control">
            	<p class="simple box round active" id="item1"><span class="arrow"></span>КНОПКА 1</p>
            	<p class="simple box round" id="item2"><span class="arrow"></span>КНОПКА 2</p>
            	<p class="simple box round" id="item3"><span class="arrow"></span>КНОПКА 3</p>
            	<p class="simple box round" id="item4"><span class="arrow"></span>КНОПКА 4</p>
            </div>
        </div>

$('#main-slider .slider-content').fadeOut(250, function(){
    $('#content'+itemNumber[4]).fadeIn(250);
});